Get Your Dog Care Questions Answered by Experts
What breed of dog is he?About how much does he weigh?Are you sure it was only 3mg?
There's nothing to worry about then.
As you can see from this link: http://www.petplace.com/drug-library/melatonin-melatonex/page1.aspx , a 3mg dose of Melatonin is a normal dose for a dog with a behavioral problem, so the fact your boy ingested that much shouldn't cause any problems with him at all!!
I hope this helps.